Balloon angioplasty has been used for the treatment of narrowed and occluded blood vessels. A frequent complication associated with the procedure is restenosis, or vessel re-narrowing. Within 3–6 months of angioplasty, restenosis occurs in almost 50 percent of patients. In order to reduce the incidence of re-narrowing, several strategies have been developed. Implantable devices, such as stents, have been used to reduce the rate of angioplasty related restenosis by about half. The use of such devices has greatly improved the prognosis of these patients. Nevertheless, restenosis remains a formidable problem associated with the treatment of narrowed blood vessels.
Restenosis associated with interventional procedures such as balloon angioplasty may occur by two mechanisms: thrombosis and intimal hyperplasia. During angioplasty, a balloon is inflated within an affected vessel thereby compressing the blockage and imparting a significant force, and subsequent trauma, upon the vessel wall. The natural antithrombogenic lining of the vessel lumen may become damaged thereby exposing thrombogenic cellular components, such as matrix proteins. The cellular components, along with the generally antithrombogenic nature of any implanted materials (e.g., a stent), may lead to the formation of a thrombus, or blood clot. The risk of thrombosis is generally greatest immediately after the angioplasty.
The second mechanism of restenosis is intimal hyperplasia, or excessive tissue re-growth. The trauma imparted upon the vessel wall from the angioplasty is generally believed to be an important factor contributing to hyperplasia. This exuberant cellular growth may lead to vessel “scarring” and significant restenosis. The risk of hyperplasia associated restenosis is usually greatest 3 to 6 months after the procedure.
Prosthetic devices, such as stents or grafts, may be implanted during interventional procedures such as balloon angioplasty to reduce the incidence of vessel restenosis. To improve device effectiveness, stents may be coated with one or more therapeutic agents providing a mode of localized drug delivery. The therapeutic agents are typically intended to limit or prevent the aforementioned mechanisms of restenosis. For example, antithrombogenic agents such as heparin or clotting cascade IIb/IIIa inhibitors (e.g., abciximab and eptifibatide) may be coated on the stent thereby diminishing thrombus formation. Such agents may effectively limit clot formation at or near the implanted device. Some antithrombogenic agents, however, may not be effective against intimal hyperplasia. Therefore, the stent may also be coated with antiproliferative agents or other compounds to reduce excessive endothelial re-growth. Therapeutic agents provided as coatings on implantable medical devices may effectively limit restenosis and reduce the need for repeated treatments.
Several considerations should be made when devising a strategy for coating implantable prosthetic devices, such as stents or grafts. One consideration in coating strategy relates to surface uniformity. Ideally, coatings should be evenly applied with limited surface imperfections. Some coating strategies, however, may produce pooling of the coating material and/or dry spots. Failure to control surface uniformity may lead to inaccurate, non-uniform drug dose delivery and therapeutic variability from device to device. Therefore, it would be desirable to provide a uniform stent coating.
Another consideration in coating strategy relates to topography. It may be desirable for the coating to be disposed on certain areas of the stent. For example, stents typically have a wire mesh with open spaces formed between. Some coating strategies may leave ‘bridged’ material within the open spaces. Such coating bridges may break off causing complications or may prevent the device from expanding or functioning properly. As another example, only certain portions of the stent may require coating, or several coating layers may be required. As such, it would be desirable to control the stent coating topography.
Another consideration in coating strategy relates to efficiency. It may be desirable to effectively coat the implantable device in relatively short time, with a minimal amount of coating material. Some coating strategies require lengthy steps, thereby reducing the amount of devices that can be coating within a certain period. In addition, some strategies do not utilize coating material in a complete manner thereby increasing cost. For example, coating material that is vaporized may get dispersed on areas other than the stent surface. Therefore, it would be desirable to efficiently coat the stent.
Accordingly, it would be desirable to provide a strategy for coating a stent that would overcome the aforementioned and other disadvantages.